Our aim is to understand mechanisms involved in globin gene control, particularly developmental switching. The human K562 cell line mimics the embryonic and fetal erythroid cell milieu and is thus useful in studying molecular mechanisms controlling the human fetal gamma-globin genes. Earlier we showed that a gamma-globin gene promoter fragment spanning positions -259 and -137 activated the nonfunctional beta-globin promoter in these cells. We therefore constructed a series of composite promoters, each containing a fragment from within this 120 base pair region joined to a beta-globin promoter, and assessed their function in K562 cells. The assays previously used, however, proved to be inadequate for fine functional mapping. Modifications are currently being developed.